The present invention generally relates to processes for producing orthopedic implants (e.g., hip, knee, shoulder replacements, etc.) having a subsurface level silicon nitride embedded layer applied via ion bombardment, and related implant products. More specifically, the present invention relates to using an ion beam to promote gas-phase reactions for subsequent relatively uniform layered implantation of reacted precipitate silicon nitride molecules into the subsurface of one or more target orthopedic implants.
Orthopedic implants (e.g., prosthetic joints to replace damaged hips, knees, shoulders, etc.) are commonly made of metal alloys such as cobalt chromium (CoCr) or titanium (Ti-6Al-4V). The mechanical properties of such metal alloys are particularly desirable for use in load-bearing applications, such as orthopedic implants. Although, when orthopedic implants are placed within the body, the physiological environment can cause the implant material to wear and corrode over time (especially articulatory surfaces), sometimes resulting in complications that require revision surgery. While hip and knee replacement surgery has been reported to be successful at reducing joint pain for 90-95% of patients, there are several complications that remain and the potential for revision surgery increases at a rate around 1% per year following a successful surgery. These complications can include infection and inflammatory tissue responses stemming from tribological debris particles from metal alloy implants, such as cobalt chromium, as a result of wear and corrosion over time.
To reduce the risk of complications from orthopedic implants, ceramic coatings have been applied to address the coefficient of friction of a wear couple, to specifically improve the surface roughness, and to reduce adhesion of a broad range of bacteria for purposes of reducing the rate of infection. For example, alumina (Al2O3) and zirconia (ZrO2) are ceramics that have been used to coat the surfaces of orthopedic implants. These ceramic materials provide high wear resistance, reduced surface roughness, and high biocompatibility. But, both materials are not optimal for the fatigue loading of non-spherical geometry of most orthopedic implants due to poor tensile strength and low toughness. Accordingly, the disadvantages of these ceramic coatings, while addressing issues related to high wear resistance and surface roughness, cannot address other failure modes such as tensile strength and impact stresses.
Conventionally, ceramic coatings such as silicon nitride have been applied to the implant surface by a chemical vapor deposition (CVD) process or a physical vapor deposition (PVD) process. In one example, a PVD process is used to coat an implant joint with an external layer of silicon nitride. More specifically, such a process includes placing the implant, a silicon-containing material, and nitrogen gas (N2) in a chamber that is heated to between 100-600 degrees Celsius. In response to the high temperatures, silicon atoms sputter from the silicon-containing material and subsequently react with the nitrogen gas at the heated surface of the implant to deposit a silicon nitride over-coat. One problem with this process is that there is no diffusion of the deposited silicon nitride molecules into the substrate material. That is, the silicon nitride is simply applied as an over-surface coating having a distinct boundary line between the deposited over-coating and the underlying substrate of the orthopedic implant. The adverse result is that the silicon nitride still experiences relatively poor surface adhesion and, over time, this over-surface coating can wear off, especially when the surface is an articulating surface (e.g., a ball-and-socket joint).
While vapor deposition of silicon nitride has been shown to work as an over-surface coating to certain orthopedic materials, such application is typically more expensive and less efficient than alumina or zirconia ceramic coatings. Moreover, it is often difficult, if not impossible, to attain a uniform application of silicon nitride to all surfaces of the orthopedic implant using known vapor deposition processes, such as those mentioned above. As a result, some areas of the over-surface coating have an undesirably thin layer of silicon nitride, wherein such areas are even more prone to reduced protection and wear. Alternatively, silicon nitride has also been used as the bulk or base material for orthopedic implants, but the production of a silicon nitride-based orthopedic implant is limited in size and inefficient to produce.
Recently, newer coating processes have been developed to provide greater adhesion by promoting diffusion of the coating material at the interface of the substrate and coating layers. Ion beam enhanced deposition (IBED), also known as ion beam assisted deposition (IBAD), is a process by which accelerated ions drive a vapor phase coating material into the subsurface of a substrate. Coatings applied by IBED may have greater adhesion than similar coatings applied by a conventional PVD process. Coatings applied by IBED may also have less delamination under impact stresses. For example, U.S. Pat. No. 7,790,216 to Popoola et al., the contents of which are herein incorporated by reference in their entirety, discloses a method of bombarding a medical implant with zirconium ions and then heating the implant in an oxygenated environment to induce the formation of zirconia (ZrO2) at the surface. In this respect, the ion beam drives the zirconium ions to a certain depth within the surface of the implant known as the “intermix zone”. Heat treatment within the oxygenated environment results in an embedded zirconia surface layer of approximately 5 micrometer (μm) thickness. The zirconia surface layer effectively penetrates the substrate and thereby resists delamination. But, this production method can be inefficient due to the high energy requirement for the heat treatment step. Likewise, the mechanical properties of the zirconia surface layer formed are not as desirable as those of a silicon nitride surface layer, which is incompatible with a heat treatment step.
There exists, therefore, a need in the art for processes for producing orthopedic implants having a subsurface silicon nitride layer applied via ion bombardment that provides greater integration of silicon nitride into the implant, thereby providing greater resistance to the emission of tribological debris. Such processes may include placing an orthopedic implant in a vacuum chamber, diffusing gaseous nitrogen and silicon within the chamber, and bombarding a surface of the orthopedic implant with an ion beam sufficient to promote gas-phase reactions to form silicon nitride and drive reacted precipitate silicon nitride molecules into the subsurface of the medical implant. The present invention fulfills these needs and provides further related advantages.